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.TH "BTRFS-SCRUB" "8" "Mar 10, 2024" "6.7.1" "BTRFS"
.SH NAME
btrfs-scrub \- scrub btrfs filesystem, verify block checksums
.SH SYNOPSIS
.sp
\fBbtrfs scrub\fP <subcommand> <args>
.SH DESCRIPTION
.sp
Scrub is a pass over all filesystem data and metadata and verifying the
checksums. If a valid copy is available (replicated block group profiles) then
the damaged one is repaired. All copies of the replicated profiles are validated.
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
Scrub is not a filesystem checker (fsck) and does not verify nor repair
structural damage in the filesystem. It really only checks checksums of data
and tree blocks, it doesn\(aqt ensure the content of tree blocks is valid and
consistent. There\(aqs some validation performed when metadata blocks are read
from disk (\fI\%Tree checker\fP) but it\(aqs not extensive and cannot substitute
full \fI\%btrfs\-check(8)\fP run.
.UNINDENT
.UNINDENT
.sp
The user is supposed to run it manually or via a periodic system service. The
recommended period is a month but it could be less. The estimated device bandwidth
utilization is about 80% on an idle filesystem.
.sp
The scrubbing status is recorded in \fB/var/lib/btrfs/\fP in textual files named
\fIscrub.status.UUID\fP for a filesystem identified by the given UUID. (Progress
state is communicated through a named pipe in file \fIscrub.progress.UUID\fP in the
same directory.) The status file is updated every 5 seconds. A resumed scrub
will continue from the last saved position.
.sp
Scrub can be started only on a mounted filesystem, though it\(aqs possible to
scrub only a selected device. See \fI\%btrfs scrub start\fP for more.
.SS Bandwidth and IO limiting
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
The \fBionice(1)\fP may not be generally supported by all IO schedulers and
the options to \fBbtrfs scrub start\fP may not work as expected.
.UNINDENT
.UNINDENT
.sp
In the past when the \fI\%CFQ IO scheduler\fP was generally used
the \fBionice(1)\fP syscalls set the priority to \fIidle\fP so the IO would not
interfere with regular IO. Since the kernel 5.0 the CFQ is not available.
.sp
The IO scheduler known to support that is \fI\%BFQ\fP, but first read the
documentation before using it!
.sp
For other commonly used schedulers like \fI\%mq\-deadline\fP it\(aqs recommended to use
\fIcgroup2 IO controller\fP which could be managed by e.g. \fIsystemd\fP
(documented in \fBsystemd.resource\-control\fP). However, starting scrub like that
is not yet completely straightforward. The IO controller must know the physical
device of the filesystem and create a slice so all processes started from that
belong to the same accounting group.
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
$ systemd\-run \-p \(dqIOBandwidthReadMax=/dev/sdx 10M\(dq btrfs scrub start \-B /
.ft P
.fi
.UNINDENT
.UNINDENT
.sp
Since linux 5.14 it\(aqs possible to set the per\-device bandwidth limits in a
BTRFS\-specific way using files \fB/sys/fs/btrfs/FSID/devinfo/DEVID/scrub_speed_max\fP\&.
This setting is not persistent, lasts until the filesystem is unmounted.
Currently set limits can be displayed by command \fI\%btrfs scrub
limit\fP\&.
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
$ echo 100m > /sys/fs/btrfs/9b5fd16e\-1b64\-4f9b\-904a\-74e74c0bbadc/devinfo/1/scrub_speed_max
$ btrfs scrub limit /
UUID: 9b5fd16e\-1b64\-4f9b\-904a\-74e74c0bbadc
Id      Limit      Path
\-\-  \-\-\-\-\-\-\-\-\-  \-\-\-\-\-\-\-\-
 1  100.00MiB  /dev/sdx
.ft P
.fi
.UNINDENT
.UNINDENT
.SH SUBCOMMAND
.INDENT 0.0
.TP
.B cancel <path>|<device>
If a scrub is running on the filesystem identified by \fIpath\fP or
\fIdevice\fP, cancel it.
.sp
If a \fIdevice\fP is specified, the corresponding filesystem is found and
\fBbtrfs scrub cancel\fP behaves as if it was called on that filesystem.
The progress is saved in the status file so \fBbtrfs scrub resume\fP can
continue from the last position.
.UNINDENT
.INDENT 0.0
.TP
.B limit [options] <path>
Show or set scrub limits on devices of the given filesystem.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B \-d|\-\-devid DEVID
select the device by DEVID to apply the limit
.TP
.B \-l|\-\-limit SIZE
set the limit of the device to SIZE (size units with suffix),
or 0 to reset to \fIunlimited\fP
.TP
.B \-a|\-\-all
apply the limit to all devices
.UNINDENT
.INDENT 7.0
.TP
.B  \-\-raw
print all numbers raw values in bytes without the \fIB\fP suffix
.TP
.B  \-\-human\-readable
print human friendly numbers, base 1024, this is the default
.TP
.B  \-\-iec
select the 1024 base for the following options, according to
the IEC standard
.TP
.B  \-\-si
select the 1000 base for the following options, according to the SI standard
.TP
.B  \-\-kbytes
show sizes in KiB, or kB with \-\-si
.TP
.B  \-\-mbytes
show sizes in MiB, or MB with \-\-si
.TP
.B  \-\-gbytes
show sizes in GiB, or GB with \-\-si
.TP
.B  \-\-tbytes
show sizes in TiB, or TB with \-\-si
.UNINDENT
.TP
.B resume [\-BdqrR] <path>|<device>
Resume a cancelled or interrupted scrub on the filesystem identified by
\fIpath\fP or on a given \fIdevice\fP\&. The starting point is read from the
status file if it exists.
.sp
This does not start a new scrub if the last scrub finished successfully.
.sp
\fBOptions\fP
.sp
see \fBscrub start\fP\&.
.UNINDENT
.INDENT 0.0
.TP
.B start [\-BdrRf] <path>|<device>
Start a scrub on all devices of the mounted filesystem identified by
\fIpath\fP or on a single \fIdevice\fP\&. If a scrub is already running, the new
one will not start. A device of an unmounted filesystem cannot be
scrubbed this way.
.sp
Without options, scrub is started as a background process. The
automatic repairs of damaged copies are performed by default for block
group profiles with redundancy. No\-repair can be enabled by option \fI\-r\fP\&.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B  \-B
do not background and print scrub statistics when finished
.TP
.B  \-d
print separate statistics for each device of the filesystem
(\fI\-B\fP only) at the end
.TP
.B  \-r
run in read\-only mode, do not attempt to correct anything, can
be run on a read\-only filesystem
.TP
.B  \-R
raw print mode, print full data instead of summary
.TP
.B  \-f
force starting new scrub even if a scrub is already running,
this can useful when scrub status file is damaged and reports a
running scrub although it is not, but should not normally be
necessary
.UNINDENT
.sp
\fBDeprecated options\fP
.INDENT 7.0
.TP
.BI \-c \ <ioprio_class>
set IO priority class (see \fBionice(1)\fP manual page) if the IO
scheduler configured for the device supports ionice. This is
only supported by BFQ or Kyber but is \fInot\fP supported by
mq\-deadline. Please read the section about
\fI\%IO limiting\fP\&.
.TP
.BI \-n \ <ioprio_classdata>
set IO priority classdata (see \fBionice(1)\fP manpage)
.TP
.B  \-q
(deprecated) alias for global \fI\-q\fP option
.UNINDENT
.TP
.B status [options] <path>|<device>
Show status of a running scrub for the filesystem identified by \fIpath\fP
or for the specified \fIdevice\fP\&.
.sp
If no scrub is running, show statistics of the last finished or
cancelled scrub for that filesystem or device.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B  \-d
print separate statistics for each device of the filesystem
.TP
.B  \-R
print all raw statistics without postprocessing as returned by
the status ioctl
.TP
.B  \-\-raw
print all numbers raw values in bytes without the \fIB\fP suffix
.TP
.B  \-\-human\-readable
print human friendly numbers, base 1024, this is the default
.TP
.B  \-\-iec
select the 1024 base for the following options, according to
the IEC standard
.TP
.B  \-\-si
select the 1000 base for the following options, according to the SI standard
.TP
.B  \-\-kbytes
show sizes in KiB, or kB with \-\-si
.TP
.B  \-\-mbytes
show sizes in MiB, or MB with \-\-si
.TP
.B  \-\-gbytes
show sizes in GiB, or GB with \-\-si
.TP
.B  \-\-tbytes
show sizes in TiB, or TB with \-\-si
.UNINDENT
.sp
A status on a filesystem without any error looks like the following:
.INDENT 7.0
.INDENT 3.5
.sp
.nf
.ft C
# btrfs scrub start /
# btrfs scrub status /
UUID:             76fac721\-2294\-4f89\-a1af\-620cde7a1980
Scrub started:    Wed Apr 10 12:34:56 2023
Status:           running
Duration:         0:00:05
Time left:        0:00:05
ETA:              Wed Apr 10 12:35:01 2023
Total to scrub:   28.32GiB
Bytes scrubbed:   13.76GiB  (48.59%)
Rate:             2.75GiB/s
Error summary:    no errors found
.ft P
.fi
.UNINDENT
.UNINDENT
.sp
With some errors found:
.INDENT 7.0
.INDENT 3.5
.sp
.nf
.ft C
Error summary:    csum=72
  Corrected:      2
  Uncorrectable:  72
  Unverified:     0
.ft P
.fi
.UNINDENT
.UNINDENT
.INDENT 7.0
.IP \(bu 2
\fICorrected\fP \-\- number of bad blocks that were repaired from another copy
.IP \(bu 2
\fIUncorrectable\fP \-\- errors detected at read time but not possible to repair from other copy
.IP \(bu 2
\fIUnverified\fP \-\- transient errors, first read failed but a retry
succeeded, may be affected by lower layers that group or split IO requests
.IP \(bu 2
\fIError summary\fP \-\- followed by a more detailed list of errors found
.INDENT 2.0
.IP \(bu 2
\fIcsum\fP \-\- checksum mismatch
.IP \(bu 2
\fIsuper\fP \-\- super block errors, unless the error is fixed
immediately, the next commit will overwrite superblock
.IP \(bu 2
\fIverify\fP \-\- metadata block header errors
.IP \(bu 2
\fIread\fP \-\- blocks can\(aqt be read due to IO errors
.UNINDENT
.UNINDENT
.sp
It\(aqs possible to set a per\-device limit via file
\fBsysfs/fs/btrfs/FSID/devinfo/scrub_speed_max\fP\&. In that case
the limit is printed on the \fIRate:\fP line if option \fI\-d\fP is specified,
or without it on a single\-device filesystem.  Read more about tat in
section about \fI\%scrub IO limiting\fP\&.
.INDENT 7.0
.INDENT 3.5
.sp
.nf
.ft C
Rate:             989.0MiB/s (limit 1.0G/s)
.ft P
.fi
.UNINDENT
.UNINDENT
.sp
On a multi\-device filesystem with at least one device limit the
overall stats cannot print the limit without \fI\-d\fP so there\(aqs a not that
some limits are set:
.INDENT 7.0
.INDENT 3.5
.sp
.nf
.ft C
Rate:             36.37MiB/s (some device limits set)
.ft P
.fi
.UNINDENT
.UNINDENT
.UNINDENT
.SH EXIT STATUS
.sp
\fBbtrfs scrub\fP returns a zero exit status if it succeeds. Non zero is
returned in case of failure:
.INDENT 0.0
.TP
.B 1
scrub couldn\(aqt be performed
.TP
.B 2
there is nothing to resume
.TP
.B 3
scrub found uncorrectable errors
.UNINDENT
.SH AVAILABILITY
.sp
\fBbtrfs\fP is part of btrfs\-progs.  Please refer to the documentation at
\fI\%https://btrfs.readthedocs.io\fP\&.
.SH SEE ALSO
.sp
\fI\%mkfs.btrfs(8)\fP
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